This invention relates to ceramics and more particularly to aluminosilicate ceramics.
Ceramics based on monoclinic celsian (BaO.multidot.Al.sub.2 O.sub.3 .multidot.2SiO.sub.2) can be considered for applications requiring materials with a combination of high melting point, low thermal expansion, high thermal shock resistance, high-frequency working capabilities, low and thermally stable dielectric constant and low loss tangent. For example, celsian is a promising candidate for use as thermally-stable dielectric and refractory materials. Celsian ceramics have been reported in the literature as having a thermal expansion of 2.5.times.10.sup.-6 /degree (at 20.degree.-1000.degree. C.), bending strength up to 110 MPa, dielectric constant and loss tangent 6-7 and (1-2).times.10.sup.-4, respectively, at 20.degree. C. and 1 MHz, and dielectric constant stable up to 600.degree. C. These ceramics were prepared from natural (kaolin or clay) or technical grade purity starting materials containing significant amounts of impurities. The impurities can adversely affect all properties, particularly the strenth and the dielectric behavior of the ceramics at high temperatures.
Celsian with melting point of about 1760.degree. C. exists in two main crystalline modifications: monoclinic, stable up to 1590.degree. C., and hexagonal, stable from 1590.degree. C. to melting temperature. Although the hexagonal modification is stable at temperatures above 1590.degree. C., it tends to be the first product of solid phase reaction and has a strong tendency to persist metastably in the whole temperature range. Hexagonal celsian transforms reversibly into low temperature orthorhombic form at 300.degree. C. This transformation is accompanied by significant volume changes. Because of this fact, hexagonal celsian is of no practical use as a ceramic material for high-temperature applications, especially in thermal cycling. Properties of celsian ceramics mentioned above belong to monoclinic modification. Literature reveals that the transformation of hexagonal celsian into the monoclinic form is promoted by prolonged high-temperature (above 1450.degree. C.) heating, hydrothermal treatment at about 2 kbar pressure, formation of glass phase during firing, and by the presence of impurities or the addition of certain additives (such as B.sub.2 O.sub.3, LiF, Cr.sub.2 O.sub.3, CaF.sub.2, ZrSiO.sub.3). However, the preparation of monoclinic celsian from high-purity raw materials free of undesirable additives by conventional processes can be only accomplished by long term high temperature treatment.
It would be desirable to provide a method of making pure monoclinic celsian at lower temperatures for shorter heating times without the use of contaminating additives.